The present invention relates generally to the drive mechanism that positions control rods within the nuclear core of a boiling water reactor. More particularly, a control rod drive which is completely contained within the reactor pressure vessel is described.
In boiling water reactors, the control rod drives are traditionally positioned outside and below the reactor pressure vessel. Since the control rods are each positioned within the pressure vessel, each drive must include a coupling that penetrates the pressure vessel. This design has several disadvantages. One of the most noticeable drawbacks is the requirement that the containment structure (as well as the reactor pressure vessel) must be very high in order to provide sufficient storage space for the control rods below the reactor fuel bundles. Another major drawback is the existence of a very large number of items that must penetrate the reactor walls (i.e. the connecting rods).